Each tool arrangement of the aforesaid kind intended for driving a high-frequency oscillating machine tool comprising one or more crystals which change their form when subjected to a change in voltage and which are incorporated in a tool holder and which utilize an electric drive circuit constructed to produce an alternating voltage which can be applied to the crystal, is fitted with a tool tip.
As a result of its construction, the material from which it is made, and its size and shape, each such tool tip, has a resonance frequency at which the energy losses are low. However, when pressure is brought to bear on the tool tip as it performs work, the resonance frequency will change momentarily and consequently there have been proposed various control circuits or regulating circuits (first control circuits) which strive to cause the high frequency oscillation or the alternating voltage applied to the crystal to lie in a momentary resonance frequency. In principle, the resonance frequency is determined electrically by two parallel-connected electrical oscillation circuits, where a first resonance circuit can be considered to constitute a capacitance and an inductance value which is dependent on the extent to which the tool tip is loaded, and therewith varies in time, and where a second resonance circuit can be considered to comprise a substantially fixed capacitance belonging to or related to the crystal and a substantially fixed inductance incorporated in the drive circuit.
It is also known that a given change in resonance frequency will occur as a result of operating conditions and when the tool tip is loaded. This change is normally of such small magnitude that the aforesaid second resonance circuit, with said fixed inductance, will still function very effectively provided that the tool tip is suitably adapted to a selected resonance frequency and provided that the operating conditions are suitably selected within narrow limits.
It has also been proposed to use a frequency controllable supply voltage whose frequency can be adapted to momentary resonance frequencies, by using a first control circuit.
Previously known constructions of this kind enable frequency to be controlled or regulated within narrow limits, say +/-2 kHz, since one control circuit, constructed to detect and regulate towards a fixed minimum impedence, can readily slide over resonance criteria (100 at FIG. 6B) and when detecting and regulating towards a prevailing phase position, between the current and voltage values, there is no positive impedence phase around the series resonance (200 and 200' at FIG. 6B and 6C).
It has been found, however, that when such a drive circuit construction is used at very high loads and/or at prevailing idling frequencies which lie adjacent the resonance frequency and at moderate load, there is risk that the resonance frequency will be changed to such an extent as to inactivate the drive circuit.
It is also known that the exchange of one tool tip for another will normally change the impedence value of the tool arrangement, such as resistance value, capacitance value and/or inductance value included in the first oscillation circuit, to such an extent that the resonance oscillation frequency will lie in the marginal range of said function or completely outside said range, resulting in a functional deficiency. By this is meant that the minimum impedence frequency is separate from the series resonance frequency, which makes it difficult to follow the changes in the series resonance frequency.
It is difficult, if not impossible, to regulate the frequency of the supply voltage with the aid of prior known techniques, such that said frequency will comply with a resonance frequency applicable to the changed resonance frequencies of the two parallel-connected oscillation circuits.
It is known that a drive circuit of the aforesaid kind, functioning to drive a tool arrangement which oscillates at resonance frequency, incorporating a fixed inductance, will permit a frequency change of about +/- 2 kHz around the resonance frequency before the requisite coaction between drive circuit and tool ceases to take place.
None of the earlier known constructions of this kind incorporate means for switching between a high-power-state, for operating the tool tip, and a low-power-state, for "stand-by" function or preparatory function.